1. Field of the Invention
The present invention relates to an induction heating device that is employed in a fuser of an image forming apparatus which thermally fixes onto a recording sheet a toner image produced thereon and that adopts particularly an electromagnetic induction technique (an IH technique) as a heating technique.
2. Description of the Related Art
A market demand for energy conservation and increasing speed of an image forming apparatus, such as a printer, a copier, and a facsimile, is recently growing. In order to fulfill these required performances, improvements in thermal efficiency of a fuser used in the image forming apparatus are important.
As described in (JP-A-2003-223063) and the like, a technique for generating Joule heat from an eddy current, which has developed in a magnetic metal component from an alternating field, and heating an element to be heated including a metal material by means of electromagnetic induction heating has already been proposed as a fuser of electromagnetic induction heating type.
In relation to the fuser of electromagnetic induction heating type, a fuser of electromagnetic induction heating type equipped with two excitation coils whose magnetic fluxes differ in direction from each other at an angle of 90° has also been proposed (JP-A-2002-341692).
Incidentally, increasing speedup of an image forming apparatus has recently been demanded, and shortening of a heating time of a fuser is sought.
However, a current value that a single system of power line can supply to one image forming apparatus is limited up to; for instance, 15 A. A conceivable method for increasing the heating speed of the fuser is to temporarily supply a large amount of electric power from the power line. However, when a commercial power source is used, the limitation makes it impossible to supply electric power of 15 A or more from the commercial power source. Accordingly, means that can temporarily supply a large amount of electric power to the image forming apparatus while avoiding supply of an overcurrent of 15 A or more from the power line is required. However, means involving laborious operation is awkward to use.
Moreover, it is desirable that a heating roller of a fuser will exhibit a uniform temperature distribution in an axial direction. However, the heating temperature declines in the vicinity of two axial ends of the heating roller. The fuser of electromagnetic induction heating type is subjected to dissipation of heat to the ambient air, and also requires a bridge area to be disposed at each end for changing a winding direction of an excitation coil to an opposite direction. A temperature fall also arises in the bridge areas. The reason for this is that, in the bridge area, a curvature radius of winding of a coil varies from one winding to another between an inner radius and an outer radius and that magnetic fluxes are not generated in a constant direction. Therefore, flux density of the windings in the bridge area becomes smaller than flux density of windings in an area other than the bridge area. As a result, a heating temperature decreases, which in turn hinders exhibition of a uniform temperature distribution in the axial direction.